1. Field of the Invention
This invention relates to a method and apparatus for acquiring image information for energy subtraction processing. This invention particularly relates to a method and apparatus for acquiring image information for two-shot energy subtraction processing, in which two kinds of images are obtained with two times of exposure to radiation.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. Also, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which causes it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
In the radiation image recording and reproducing systems wherein recording media, such as radiation film or stimulable phosphor sheets, are used, subtraction processing techniques for radiation images are often carried out on image signals detected from a plurality of radiation images of an object, which have been recorded on the recording media.
With the subtraction processing techniques for radiation images, an image is obtained which corresponds to a difference between a plurality of radiation images of an object recorded under different conditions. Specifically, a plurality of the radiation images recorded under different conditions are read out at predetermined sampling intervals, and a plurality of image signals thus detected are converted into digital image signals which represent the radiation images. The image signal components of the digital image signals, which components represent the image information recorded at corresponding sampling points (i.e., picture elements) in the radiation images, are then subtracted from each other. A difference signal is thereby obtained which represents the image of a specific structure or part of the object (hereinbelow also referred to as the pattern of a tissue, a structure, or the like) represented by the radiation images.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order for the image of a specific structure (for example, a blood vessel) of an object to be extracted from the image of the whole object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure (for example, a blood vessel) of the object is enhanced by the injection of contrast media. In the latter method, such characteristics are utilized that a specific structure of an object exhibits different levels of radiation absorptivity with respect to radiation with different energy distributions. Specifically, an object is exposed to several kinds of radiation with different energy distributions. Alternatively, the energy distribution of the radiation carrying image information of an object, is changed after it has been irradiated onto one of a plurality of radiation image recording media, after which the radiation impinges upon the second radiation image recording medium. In this manner, a plurality of radiation images are obtained in which different images of a specific structure are embedded. Thereafter, the image signals representing the plurality of the radiation images are weighted appropriately and subjected to a subtraction process in order to extract the image of the specific structure. The subtraction process is carried out with Formula (1) shown below. EQU Sproc=Ka.multidot.H-Kb.multidot.L+Kc (1)
wherein Sproc represents the subtraction image signal obtained from the subtraction process, Ka and Kb represent the weight factors, Kc represents the bias component, H represents the image signal representing the radiation image recorded with the radiation having a high energy level, and L represents the image signal representing the radiation image recorded with the radiation having a low energy level. (The group of Ka, Kb, and Kc will be referred to as the parameters for the subtraction process.)
The energy subtraction processing may be classified into two kinds of methods. One of the methods is two-shot energy subtraction processing, and the other is one-shot energy subtraction processing.
With the two-shot energy subtraction processing, wherein the difference in energy level between the two shots can be kept large, the range of the subtraction image signal obtained from the subtraction process can be kept wide. Therefore, the contrast of the subtraction image reproduced from the subtraction image signal can be kept high.
In the two-shot energy subtraction processing, the two image signals to be subjected to the subtraction process should be obtained from the two times of exposure to radiation such that the image signal components of the two image signals may accurately represent corresponding picture elements in the two images. However, actually, it is not always possible to carry out the image recording operations successively and quickly for recording the two images with the two kinds of radiation having different energy distributions. Therefore, in cases where the object is a living body, particularly the chest, in which the motion of the heart, or the like, is violent, due to the interval of time occurring between the two times of exposure, it is not always possible to obtain the two images such that the corresponding picture elements in the two images may accurately coincide with each other. As a result, the signal-to-noise ratio of an energy subtraction image, which is obtained from the energy subtraction processing, cannot be kept high.
In the one-shot energy subtraction processing, two detectors are located one upon the other with an energy separating plate, or the like, intervening therebetween, and image signals are recorded respectively on the two detectors with a single, simultaneous exposure to radiation. Therefore, the image signals recorded on the two detectors can be obtained such that they may represent the images, in which the corresponding positions accurately coincide with each other. Also, since the energy separating plate, or the like, is located between the two detectors, the image signals recorded on the two detectors can be obtained with the radiation different in energy level. However, with the one-shot energy subtraction processing, the difference in energy level between the two detectors cannot be kept large, and the range of the subtraction image signal obtained from the subtraction process cannot be kept wide. Therefore, the contrast of the subtraction image reproduced from the subtraction image signal cannot be kept high.
As described above, the conventional one-shot energy subtraction processing and the conventional two-shot energy subtraction processing have their own drawbacks.